1. Field of the Invention
This invention relates to methods and compositions for removing gelled and dehydrated drilling fluid and filter cake from the walls of wellbores.
2. Description of the Prior Art
A variety of drilling fluids are used in the drilling of wellbores. Generally, the drilling fluids are solids containing water based gels or hydrocarbon based fluids which can be non-weighted or weighted with particulate weighting materials such as barite. Most drilling fluids contain gelled bentonite and/or one or more natural and/or synthetic polymeric additives such as polysaccharides or polysaccharide derivatives. The polymeric additives add viscosity to the fluids and are included to impart sufficient carrying capacity and thixotropy whereby the fluids can transport cuttings to the surface and prevent the cuttings from settling appreciably when circulation is interrupted. Natural and synthetic polymeric fluid loss control additives are also commonly utilized in drilling fluids, e.g., polysaccharides, polysaccharide derivatives, polyacrylic acids, polyvinyl alcohol and the like.
One of the most important functions of a drilling fluid is to seal off the walls of the wellbore so that the fluid is not lost into highly permeable subterranean zones penetrated by the wellbore. This is accomplished by the deposit of a filter cake of solids from the drilling fluid, dehydrated drilling fluid and gelled drilling fluid over the surfaces of the wellbore whereby the solids bridge over the formation pores and do not permanently plug the pores.
During the drilling of a well bore, the drilling fluid is continuously circulated down the drill pipe, through the drill bit and back to the surface through the annulus between the drill pipe and the walls of the wellbore. After a wellbore has been drilled to total depth, the circulation of the drilling fluid is stopped while the well is logged and pipe is run in the wellbore. During the shut down period, additional gelled and dehydrated drilling fluid and filter cake is deposited on the walls of the wellbore. As a result of the polymeric viscosifiers and additives in the drilling fluid, the filter cake formed is generally very stable and difficult to remove.
After pipe is run in the well bore, primary cementing operations are usually performed therein. That is, the pipe is cemented in the wellbore by placing a cement slurry in the annulus between the pipe and the walls of the wellbore. The cement slurry sets into a hard impermeable mass, and it is intended to bond the pipe to the walls of the wellbore whereby the annulus is sealed and fluid communication between subterranean zones or to the surface by way of the annulus is prevented.
In order for the cementing of the pipe in the wellbore to be successful, the gelled and dehydrated drilling fluid and filter cake deposited on the walls of the wellbore must be removed. If appreciable drilling fluid and filter cake remain on the walls of the well bore, the cement will not properly bond thereto and fluid leakage through the annulus and other major problems will result.
Heretofore, attempts have been made to remove the drilling fluid and filter cake from the wellbore by attaching mechanical scrapers to the pipe so that as the pipe is run into the well bore, it physically contacts and breaks up some of the drilling fluid and filter cake. In addition, flushes are normally run through the annulus between the pipe and the walls of the wellbore prior to cementing in order to remove drilling fluid and filter cake therein. Such flushes generally provide only minimal drilling fluid and filter cake removal. Other improvements have achieved better results such as pumping fluids through the annulus at high rates so that they are in turbulence as they contact the filter cake, and including surfactants in flush fluids to lower surface tension and enhance the penetration of the flush fluids into the filter cake.
While the above described methods and other prior art techniques have achieved varying degrees of success, there is a need for improved methods of removing stable polymer-containing filter cake from the walls of a wellbore in a relatively short period of time prior to or as a part of primary cementing operations whereby the filter cake is substantially removed and successful cementing is achieved.